This invention relates to a plasma (inductively-coupled or microwave induced) mass spectrometer, and in particular to such a spectrometer intended for the determination of isotopic ratios.
Two of the most significant problems which limit the performance of prior plasma mass spectrometers are firstly, the very low efficiency of transfer of the ions generated in the plasma through the interface into the vacuum system containing the mass analyzer, and secondly, the presence of interfering ion signals, sometimes very intense, due to species generated in the plasma other than the atomic ions characteristic of the elements present in a sample. These interfering ion species comprise atomic or molecular ions such as Ar+, Ar++, ArH+, ArN+ etc. which are generated by the plasma in the absence of any introduced sample, and also molecular ions such as oxides, argides and hydride ions formed by reaction of the elements present in a sample with other species present in the sample. Not only do some of these interfering ions mask the signals from atomic ions for which a measurement is required because they have the same mass-to-charge ratio as that of an atomic ion to be measured, but they also result in a very high total ion current, much greater than that typically available from a sample. The maximum ion current that can be transmitted through any ion-optical system is generally limited by space-charge effects, and in practice the high ion current due to these unwanted species can saturate the spectrometer optics, reducing the number of sample ions transmitted and causing other undesirable effects such as mass discrimination and matrix effects.
Considerable research effort has been expended in trying to reduce both the quantity and the deleterious effect of these interfering ions, and the following is a review of that work relevant to the present invention. Rowan and Houk (Appld. Spectroscopy, 1989 vol 43(6) pp 976-980) and Rowan (Thesis, Iowa State University, submitted 1989) describe a failed attempt to reduce the number of polyatomic ions entering the mass analyzer of a plasma mass spectrometer by collision-induced dissociation. An RF-only quadrupole was disposed between the nozzle-skimmer interface and the mass-analyzing quadrupole of an otherwise conventional ICP mass spectrometer, and a collision gas, (typically xenon) was introduced into it at a pressure between 10xe2x88x925 and 10xe2x88x924 torr. It was hoped that this would induce dissociation of unwanted polyatomic species before they entered the mass analyzer by a mechanism similar to the collisional dissociation of molecular ions used in the triple quadrupole mass spectrometers intended for use in organic mass spectrometry. Although Rowan and Houk were able to demonstrate an improvement in the ratio of wanted to unwanted ions by this technique, the ion transmission efficiency of the instrument was greatly reduced and the intensity of the background signals increased, so that they concluded that any beneficial effect was in general outweighed by the disadvantages.
A similar approach was reported by Douglas (Can. J. Spectroscopy, 1989 vol 34(2) pp 38-49, in particular the passage bridging pp 47-48). In this work a triple quadrupole spectrometer was fitted with an ICP source with the aim of dissociating unwanted polyatomic ions in the centre quadrupole. This approach also failed, and Douglas predicted that it would not be possible to achieve large gains in the atomic ion to polyatomic ion ratio by collision-induced dissociation because the loss cross-sections for the atomic ions were found to be much higher than expected; so much higher, in fact, that they were comparable to those of the polyatomic ions. Thus the net effect of the collision process would be to cause roughly equal losses of both atomic and polyatomic ions. Douglas concludes that a more profitable approach might be to use ion-molecule chemistry in the centre quadrupole (that is, to chemically convert both wanted and unwanted ions, for example by reaction with oxygen) to species such as oxides. Certain polyatomic species generated in the plasma, for example oxides, would then be less likely to undergo further reaction, so that the ratio of reacted atomic ions to reacted polyatomic ions would in some cases be reduced. However, this approach is obviously highly specific and while reducing the effect of one interfering ion may introduce another that was not previously present.
Also in 1989, King and Harrison (Int. J. Mass Spectrom. And Ion Proc, 1989 vol. 89 pp 171-185) described the use of collision-induced dissociation to remove polyatomic ion interferences in glow-discharge mass spectrometry. Like Douglas, they employed a triple quadrupole mass spectrometer and used the centre quadrupole as a collision cell. Their results were similar to those of Rowan and Houk with an ICP spectrometer, namely, that although it was possible to demonstrate a reduction in the ratio of certain polyatomic ions to wanted atomic ions, the ion transmission was severely reduced, causing an overall reduction in detection limits.
Presumably because of the failure of the work in 1989 to demonstrate a worthwhile reduction in polyatomic ion interferences in ICPMS, and Douglas""s comments that this was to be expected on theoretical grounds, research effort related to reducing interferences switched to development of other aspects of ICPMS, and it was not until 1996 that Eiden, Barinaga and Koppenaal (J. Anal. Atomic. Spectrom., 1996 vol 11 pp 317-322) described a method for the selective removal of plasma matrix ions such as Ar+ from either an ion-trap ICP spectrometer or from the ion beam in a quadrupole ICP mass spectrometer by the reaction of added gaseous hydrogen with the ions sampled from the plasma. In practice, hydrogen was introduced into the vacuum system of the spectrometer downstream of the conventional nozzle-skimmer system (which is used to interface the plasma to the mass analyzer) at a pressure of about 10 mtorr, and it was found that Ar+ ions were removed 45 times faster than typical atomic ions, leading to a large reduction in the intensity of the Ar+ peak in a typical mass spectrum. The results were more spectacular in the case of an ion-trap spectrometer, leading to almost complete elimination of the Ar+ peak. Eiden et.al. also suggest that the efficiency of the removal of Ar+ in a quadrupole mass spectrometer might be increased by using a radio-frequency quadrupole ion guide (or other multipole device), into which hydrogen is introduced, between the skimmer and the mass analyzer. They suggest that operating the quadrupole guide with a low-mass cut-off of between 5 and 15 daltons might reject charged hydrogen ions generated by chemical reaction between the added hydrogen and the unwanted Ar+ ions, thereby minimising the number of charged species passing into the mass analyzer and consequently reducing space-charge related problems. However, the method is dependent on chemical reaction between the added hydrogen and the unwanted ions, and similar reactions may take place between the hydrogen and the atomic ions to be determined, albeit at a much slower rate, generating unwanted mass discrimination effects and additional molecular ions. Because the removal of ions is a chemical process, Eiden, et al, do not teach that any gas other than hydrogen could be used.
Further art relevant to this invention is typified by U.S. Pat. No. 4,963,736 which teaches an atmospheric pressure ionization (API) quadrupole mass spectrometer in which an AC-only multipole (i.e., quadrupole or hexapole, etc) rod set is disposed between the API source and the quadrupole mass filter. Gas is introduced into the vacuum system in the vicinity of the additional rod set. The inventors claim that this results in improved mass resolution of the quadrupole mass analyzer and a narrow range of energies of the ions emerging from the additional rod set. More details of this technique were later published by the inventors (Douglas and French) in J.Am. Soc. Mass Spectrom., 1992, vol 3 pp 398-408. However, neither the patent or the subsequent paper teach or even suggest that the collisional focusing which it describes could advantageously be employed in the case of a plasma mass spectrometer having an ICP or MIP source.
Other prior art relating to the field of the present invention includes WO 95/23018 which teaches a variety of multipolar ion guides for transporting ions through one or more pressure reduction stages between the ion source and the mass analyzer of a mass spectrometer. These rod sets extend from a first region maintained at a first pressure into a second region maintained at a second pressure. The multipolar rod sets may comprise 4,6, or 8 electrodes and the pressure in the space inside them may be in the range taught by U.S. Pat. No. 4,963,736, at least along part of their length. WO 95/23018 also suggests that its multipolar rod sets may be used in conjunction with an ICP source, but does not teach the use of a rod set whose entrance and exit are disposed in the same region and maintained at substantially the same pressure.
In the following, the term xe2x80x9cplasma mass spectrometerxe2x80x9d is used to describe mass spectrometers having either microwave-induced (MIP) or inductively-coupled (ICP) plasma ion sources operating substantially at atmospheric pressure, and the word xe2x80x9cplasmaxe2x80x9d means either an ICP, MIP, or glow discharge.
It is an object of the invention to provide a plasma mass spectrometer in which the interference from Ar+ and other ions generated in the plasma itself in the absence of any introduced sample is greatly reduced. It is another object to provide plasma mass spectrometers having greater mass resolution and higher ion-transmission efficiency than prior types with comparable mass analyzers. It is a further object to provide a magnetic sector plasma mass spectrometer for the determination of isotopic ratios which is less expensive and simpler than prior types of double-focusing plasma mass spectrometers.
In accordance with these objectives there is provided a mass spectrometer comprising:
1) means for generating ions from a sample introduced into a plasma;
2) nozzle-skimmer interface means for transmitting at least some of said ions from said plasma into a first evacuated chamber along a first axis;
3) diaphragm means comprising an aperture, said diaphragm means dividing said first evacuated chamber from a second evacuated chamber;
4) ion guiding means disposed in said first evacuated chamber for guiding ions from said nozzle-skimmer interface means to said aperture; and
5) ion mass-to-charge ratio analyzing means having an entrance axis and disposed to receive ions passing through said aperture and to produce a mass spectrum thereof;
said mass spectrometer being characterised in that said ion guiding means comprises:
1) one or more multipole rod-sets, the or each set comprising a plurality of elongate electrode rods spaced laterally apart a short distance from each other about a second axis to define an elongate space therebetween extending longitudinally through such set;
2) means for applying an AC voltage between rods comprised in the or each set such that ions entering said set travel in said elongate space through said rod set; and
3) means for introducing into said ion guiding means an inert gas selected from the group comprising helium, neon, argon, krypton, xenon and nitrogen so that the partial pressure of said inert gas in at least a portion of said elongate space inside said rod set(s) is at least 10xe2x88x923 torr.
Preferably, helium is introduced into said ion guiding means.
Further preferably, at least a portion of said ion guiding means is surrounded by gas containment means disposed wholly within said first evacuated chamber and disposed so that both the entrance and exit of the ion guiding means are outside of it. Said inert gas may then be introduced into said containment means. In this way a partial pressure of at least 10xe2x88x923 torr can be maintained in at least a portion of the ion guiding means while its entrance and exit are maintained at a lower pressure (typically that of the first evacuated chamber). Preferably the gas containment means is shorter than the ion guiding means and is disposed so that its longitudinal centre is closer to the entrance of the guiding means than to the exit. Typically, the length of the gas containment means may be 50% or less of the length of the ion guiding means. The inert gas should be introduced into the gas containment means so that the highest partial pressure of inert gas in the ion guiding means is located between its entrance and a point half-way along its length. A point about one-third of the length from the entrance is most preferred. The best results are obtained when the gas containment means is disposed with one end just downstream of the entrance of the ion guiding means.
Further preferably, the gas containment means should be such that a partial pressure of at least 10xe2x88x923 torr of inert gas can be maintained within it while the pressure in the first evacuated chamber is maintained at less than 10xe2x88x924 torr. The inventors have found that it is particularly advantageous to maintain the pressure at the exit of the guiding means as low as possible, and this is facilitated by use of a gas containment means which is shorter than the guiding means and is located towards the entrance, rather than the exit, of the guiding means.
The ion guiding means preferably comprises a hexapole rod set, but quadrupole or octupole sets may be used instead. It has been found that a hexapole set results in only a minimal variation in ion transmission efficiency with mass-to-charge ratio, which is especially important if isotopic ratios are to be determined. Conveniently, the length of the rod set is between 20 and 100 times greater than the radius of the elongate space between the rods, and most preferably about 50 times. The elongate rods may conveniently be of constant diameter and be disposed parallel to one another, but the use of electrode rods which are tapered and/or not parallel to each other is also within the scope of the invention. Further, an axial potential gradient may be provided along the ion guiding means which can assist ion transmission. This can be done, for example, by providing an ion guiding means which comprises a plurality of multipole rod sets disposed one after the other, with each portion having a different axial potential, or by splitting the gas containment means which surrounds the ion guiding means into several segments insulated from one another and applying different DC potentials to the segments, but other methods are also possible.
Although the rods comprising the ion guiding means are preferably supplied only with an AC voltage, it is also within the scope of the invention to add a DC potential in the manner conventional for quadrupole mass analysers, particularly if a quadrupole arrangement is employed.
In a further preferred embodiment the first axis (of the nozzle-skimmer interface means) does not pass through and the aperture in the diaphragm, so that there exists no line-of-sight path along the first axis to the aperture. The ion-guiding means is disposed so that the second axis is inclined to the first axis so that ions leaving the nozzle-skimmer interface means enter the elongate space in the guiding means and are guided by the ion confining action of the guiding means to the aperture. In this way neutral molecules or atoms are prevented from passing into the aperture and into the ion mass-to-charge analyzing means and background signals can be minimised.
In addition, a further reduction in background can be obtained by arranging the entrance axis of the mass analyzer (which receives the ions from the ion guiding means which have passed through the aperture in the diaphragm means) to be inclined relative to the second axis (of the ion guiding means). Conveniently, by inclining the second axis to both the first axis and the entrance axis, the first and entrance axes can be arranged parallel to one another, which facilitates the construction of an instrument.
In further preferred embodiments the ion mass-to-charge analyzing means comprises a magnetic sector mass analyzer. For the purposes of isotopic ratio measurements, the analyzer may be fitted with a plurality of ion collectors disposed along its image focal plane so that ions of several different mass-to-charge ratios can be measured simultaneously. Such multi-collector systems are conventional in magnetic sector isotope ratio mass spectrometers. Surprisingly, the inventors have found that it is unnecessary to use a double-focussing mass analyzer (i.e., one incorporating an electrostatic ion-energy analyzer) for this purpose because the mass resolution and abundance sensitivity of a spectrometer according to the invention is very much greater than that of a prior single-focusing plasma spectrometer with a comparable magnetic sector analyzer, but if very high resolution is required, a double-focusing analyzer could be used.
In alternative preferred embodiments, the ion mass-to-charge ratio analyzer may comprise a quadrupole mass analyzer. Such an embodiment provides an ICP mass spectrometer which is capable of analyzing atomic species which yield ions at mass-to-charge ratios where significant interferences occur with prior quadrupole instruments without the expense of a high resolution mass analyzer. In yet another preferred embodiment, the ion mass-to-charge ratio analyzer may comprise a time-of-flight analyzer, particularly one having an orthogonal disposition of the entrance axis and the ion drift direction. Such an instrument typically exhibits greater sensitivity than a quadrupole based instrument.
It is also within the scope of the invention to employ a quadrupole ion-trap or an ion cyclotron resonance mass analyzer as the ion mass-to-charge-ratio analyzer.
The inventors have surprisingly found that in a spectrometer according to the invention, ions such as Ar+ and ArX+ (where X=H, C, O, N, Cl, or Ar, etc) are very greatly reduced in intensity. This is in contrast with the work of Eiden et al. who observed suppression only as a consequence of the use of hydrogen alone and in the absence of a guiding means, and ascribed the suppression to the removal of argon ions by chemical reaction with hydrogen. Such a mechanism is clearly not possible when an inert gas is used.
It has also been found that in a spectrometer according to the invention, the mass resolution and abundance sensitivity of the ion mass-to-charge ratio analyzer is greatly improved in comparison with prior spectrometers. In contrast with the arrangement taught in U.S. Pat. No. 4,963,736 for an API source, the improvements are most marked when the second axis (of the ion-guiding means) is inclined to both the first axis (of the nozzle-skimmer interface) and entrance axis of the mass analyzer, so that no line-of-sight path exists along the nozzle-skimmer axis to the entrance aperture of the analyzer
Viewed from another aspect the invention provides a method of mass spectrometric analysis of a sample comprising the following steps carried out sequentially:
1) introducing a said sample into a plasma to generate ions therefrom;
2) passing at least some of said ions through nozzle skimmer interface means into a first evacuated chamber;
3) guiding at least some of the ions entering said first evacuated chamber to an aperture in a diaphragm which divides said first evacuated chamber from a second evacuated chamber; and
4) mass analyzing at least some of the ions passing into said second evacuated chamber to produce a mass spectrum thereof;
said method being characterised in that:
1) the step of guiding said ions comprises passing said ions through ion guiding means comprising one or more multipole electrode rod sets which comprise a plurality of elongate rod electrodes spaced laterally apart a short distance from each other to define an elongate space therebetween which extends longitudinally through the set, and applying an AC voltage to said rod electrodes; and
2) introducing into said guiding means an inert gas selected from the group comprising helium, neon, argon, krypton, xenon and nitrogen so that the partial pressure of said inert gas in at least a portion of said elongate space is at least 10xe2x88x923 torr.
In the case of a quadrupole or quadrupole ion-trap mass analyser, further advantage is obtained by maintaining only a very low potential difference between the potential of the second axis and the potential of the central axis of a subsequent quadrupole mass analyzer or the potential of the centre of a subsequent ion trap. With the gas in the ion guiding means at room temperature, this potential difference should be approximately 1 volt (with the axial potential of the ion guiding means more negative than the mass analyzer, for the case of positive ions). The potential difference is very critical and may be adjusted for maximum ion transmission. If it is too high, no ions will have sufficient energy to cross the potential barrier and enter the mass analyzer. The inventors have also discovered that adjustment of this potential provides a means of further reducing the interferences due to molecular ions generated in the plasma. It has been observed that as the potential is increased from slightly above zero towards the cut-off potential mentioned above, the intensity of the molecular ions such as argides and oxides is reduced significantly before the intensity of the atomic ions is affected.
This is unexpected because following the teachings of U.S. Pat. No. 4,963,736 it would be expected that the energy of the ions passing through the ion guiding means would in all cases become that of the thermal energy of the gas in the ion-guiding means. It appears, however, that the energy acquired by the molecular ions passing through the guiding means is slightly lower than that acquired by the atomic ions, so that adjusting the potential through which the ions must travel can effectively prevent molecular ions reaching the mass analyzer. The invention therefore further provides a method as previously defined wherein the step of mass analyzing said ions comprises the use of a quadrupole mass analyser having a central axis and the step of guiding said ions comprises passing ions through ion guiding means having a central axis, said method further comprising the step of maintaining a potential difference between the potential of the central axis of said ion guiding means and the potential of the central axis of said quadrupole mass analyser such that the transmission of polyatomic ions is reduced relative to that of atomic ions. Alternatively, the invention provides a method as previously defined wherein the step of mass analyzing said ions comprises the use of a quadrupole ion-trap mass analyser having a centre and the step of guiding said ions comprises passing ions through ion guiding means having a central axis, said method further comprising the step of maintaining a potential difference between the potential of the central axis of said ion guiding means and the potential at the centre of said quadrupole ion-trap mass analyser such that the transmission of polyatomic ions is reduced relative to that of atomic ions. In this way the invention provides a method of reducing molecular ion interferences in plasma mass spectroscopy carried out in a spectrometer as defined above. Typically this potential difference is in the range 0xc2x11 volt and is critical to a few tenths of a volt.
It will be appreciated that in the case of a magnetic sector mass analyzer it is necessary to accelerate the ions before they enter the magnetic sector to a high kinetic energy. Conventionally this is done by maintaining the ion source at a high positive potential and grounding the entrance aperture of the analyzer and all the subsequent components. However, in a spectrometer according to the invention, it is preferred to maintain the nozzle-skimmer interface and ion-guiding means at approximately ground potential. This necessitates maintaining the entrance aperture, flight-tube and detector system of the spectrometer at a high negative potential so that the ions acquire the necessary kinetic energy for dispersion by the magnetic sector as they pass through the entrance aperture. It is within the scope of the invention, however, to maintain the nozzle-skimmer interface and ion-guiding means at a high positive potential and to maintain the flight tube and detector system at ground potential.
In a still further preferred embodiment, electrostatic lens means are provided between the nozzle-skimmer interface and the entrance of the ion-guiding means. Typically this lens means is maintained at a potential of between 600 and 1000 volts negative (in the case of positive ions) relative to the potential of the nozzle-skimmer interface and the ion guiding means. Preferably the electrode comprises a hollow conical structure disposed with its apex closest to the skimmer. The lens electrode may also serve as a second diaphragm to define an additional evacuated chamber and therefore provide an additional stage of differential pumping between the nozzle-skimmer interface and the ion-guiding means. The potential applied to the electrostatic lens means is adjusted to improve the transmission efficiency of ions from the nozzle-skimmer interface to the ion guiding means. The inventors have found that when the potential is correctly set, the lens means increases the transmission efficiency by more than a factor of 100, particularly of the ions of low mass-to-charge ratio which in its absence are most likely to be lost because of space-charge effects in the vicinity of the skimmer. It has also been found that the provision of the lens reduces the transmission of ions such as ArO+, consequently improving the detection sensitivity for Fe. Use of this lens also greatly reduces mass discrimination in the nozzle-skimmer interface region, which is especially valuable when isotopic ratios are to be determined.
As in most prior plasma spectrometers, samples to be analyzed may be introduced into the plasma in the form of an aerosol generated by a conventional nebulizer. The inventors have found that best results are obtained when samples are in the form of aqueous solutions.
It has also been found that the addition of small amounts (less than 5%, and most preferably less than 1%) of another material to the inert gas can further enhance performance. For example, the addition of 0.5% of xenon to a helium inert gas surprisingly has been found to further reduce the intensity of oxygenated molecular ions, and approximately 0.5% of hydrogen or water can result in a further reduction of ions such as Ar+.